The present invention is a solar collector that reflects and concentrates sunlight to a central tower-mounted receiver. It may be used, for example, with a thermal receiver to generate heat to be stored and later used to generate electricity after sunset. Such dispatchable generation of electricity may have applications both on and off-grid, as a complement to daytime photovoltaic generation.
In the past, three principal geometries have been used to concentrate sunlight for thermal generation: steerable dish, trough, and heliostats around a central tower. Of the three geometries, dish collectors have the highest optical efficiency (lowest obliquity losses) as the reflector surface always faces the sun directly. The receiver is mounted at the focus of the dish and moves with the tracker. However, dish collectors are not always cost effective. They may have high structural costs because the structure must remain undistorted as it changes orientation tracking the sun, and may, in some instances, also need to withstand rare gale force winds. Dish collectors are in addition limited to relatively small reflector aperture.
Trough reflectors only focus light in one axis, and are thus limited to relatively low concentration.
Large central receiver plants may have a field of heliostat reflectors positioned on the ground around a tower mounted receiver. Each heliostat mirror is turned in two axes to direct sunlight to the receiver. Such collectors have deficiencies in collection efficiency, field efficiency and degree of concentration.
In the case of heliostat reflectors, collection efficiency, as measured by power concentrated per unit area of mirror collector, depends on how any given heliostat is oriented at given time of day, and is reduced for those heliostats oriented with high obliquity loss. Obliquity is the ratio of the area of sunlight reflected by the heliostat to its full mirror area. It is high for a heliostat when the shadow of the receiver falls near it, but low when a heliostat lies between the sun and tower.
Field efficiency is measured by the ratio of mirror to ground area. Heliostats located near the edge of the field to increase concentration must be spaced well apart to avoid self-shadowing, thus reducing overall field efficiency. Increasing the areal density of the field causes the receiver to see a better-filled solid angle, increasing concentration, but this high density increases shadowing losses.
The degree of concentration achievable on the central receiver is also limited by heliostats at the edge of the field. Even for the ideal case when each heliostat mirror is curved to focus a solar image on the receiver, the outer heliostats will form a larger solar image than inner-field heliostats. The receiver size must thus be increased, and the average concentration decreased to accommodate the largest image produced by the most distant heliostats.
In the past, attempts have been made to overcome some of the above limitations. For example, U.S. Patent publication No. 20120325313, to Cheung, et al., and U.S. Pat. No. 9,029,747, to Osello, are directed to systems with mobilized heliostats on circular tracks about a central tower receiver, driven so as to minimize obliquity losses. The heliostats are moved around the tracks to maintain the same azimuthal geometry relative to each other, so that each heliostat needs only motion about a single additional axis to direct sunlight to the tower. U.S. Patent publication No. 2014/0116419, to Ruiz Hernandez, is directed to a system in which heliostats are also driven around circular tracks through the day, clustering opposite the sun, and a central receiver rotated to face the heliostats. These systems reduce obliquity loss, but do not overcome the above limits to field efficiency and optical concentration common to all systems operated with heliostats near ground level.
There thus remains a need for a central receiver system with high optical concentration, high field efficiency, and low self-shadowing losses. Solar collection high optical concentration is valuable for operating receivers at high temperature, required to increase the thermodynamic limit to conversion efficiency. High temperature receivers (>600 degrees C.) have been developed to generate electricity using efficient Rankine and Brayton cycles. At the same time, the collector field preferably combines high concentration with low obliquity loss, low self-shadowing losses and inexpensive mechanical structure. An advance over the present state of the art is needed.